Nano-particles such as quantum dots (QD) are a promising phosphor substitute for narrowband red emission leading to an increase in lumen/Watt at high color rendering index. In particular, QDs can be fabricated to have a high photoluminescence quantum yield (PLQY) and may be applicable as down-converting materials in solid state lighting applications. In this way, the performance, efficiency and color choice in lighting applications, particularly light emitting diodes (LEDs), can be improved.
However, in order to apply QDs as a phosphor substitute, it is essential that degradation of the QDs is minimized. In particular, when QDs are applied directly on LEDs and used as a color converter without or only with limited additional encapsulation to protect them against external influences such as moisture, the properties of the QDs will be affected by the moisture leading to degradation of the QDs and consequently a deterioration of the lighting performance.
Different device structures are known in which the stability of QDs in converting light is improved, so that the LEDs comprising QDs as a phosphor substitute may have high lighting performance. However, the stability of light conversion of the QDs cannot be sufficiently improved in such known devices, in particular under high humidity conditions, such as in the case of Wet High Temperature Operating Life (WHTOL) tests.